Internal combustion engines contain multiple cylinders. Exhaust gas is generated when a fuel and air mixture is ignited and expanded within a cylinder to drive a piston. The exhaust gas is typically vented from the cylinders through an exhaust stroke to the atmosphere. The exhausted gas typically has a very high temperature when leaving the cylinders. In some proposed systems, the exhaust gas is delivered to a second cylinder for further expansion.
Some internal combustion engines have injected water into the same cylinder performing combustion with fuel and air intake.
There has also been a proposal for a combined engine that has a combustion cylinder mounted upstream of an expansion cylinder. The expansion cylinder receives hot exhaust gas from the combustion cylinder, and also receives a source of water that is expanded into steam by the hot exhaust gas to create further drive for a common crankshaft.
While this proposed system has good potential, there are many improvements that would make the system more practical.